Polymeric materials such as a homopolymer or copolymer of vinyl, and products made therefrom, must exhibit resistance to degradation if they are to have any useful commercial value, particularly for outdoor use, such as in house siding, window profiles such as main frame, sash, glazing beads, and the like. The degradation referred to can be a partial or total loss of structural integrity, color change of the product, such as lightening or darkening, a loss of flexibility or resilience, loss of shape due to softening, or a combination of any of these phenomena. The various types of degradation, above referred to, are promoted or catalyzed by oxygen (air), heat, and light, particularly sunlight in the ultraviolet (UV), visible, and infrared (IR) ranges.
In order to protect polymeric materials, it has been the practice heretofore to add various ingredients or stabilizers to the polymers, or compounds thereof, to prevent or inhibit degradation of finished articles made therefrom. These stabilizers work in diverse and complex ways, such that a compound that stabilizes against heat and oxygen degradation in a polymer or resin may not stabilize the same against light degradation, or vice versa. In many instances, to obtain optimum protection, a mixture of compounds, each specifically selected to afford maximum protection against a certain type of degradation, is often used.
It is well known that absorption of light energy from the sun causes a temperature rise in articles exposed to the sun. This is a particularly bad problem in the case of vinyl house siding since it causes "oil canning" of the siding which is a surface distortion. Vinyl and other plastic materials require ultraviolet absorbers for protection during weathering. These absorbers may be organic materials, titanium dioxide, carbon black, etc. However, by absorbing the ultraviolet light, the articles are heated by the ultraviolet light. Also, pigments which absorb or reflect in the visible light region are sometimes added to give color. However, these pigments absorb the visible light and heat the article by this absorbed light. To change the amount of light absorbed in the visible light region would change the color.
It would be particularly desirable to have color pigments, or other materials, which would reflect infrared energy and which would lower the heating of the article without changing the ultraviolet protection or the color thereof. In particular, it would be desirable to have a composition that is resistant to color change.
The art of compounding vinyl polymers is a very complex art which involves the "trade-off" of certain properties. That is, to improve one property, one must ordinarily take a reduction in another important property. It would be particularly desirable to be able to have improved color retention without sacrificing other important properties of a vinyl polymer compound.
Tin mercaptide stabilizers are very good for heat stability but the sulfur present causes poor light stability. Tin maleate stabilizers are known to give good light stability however they produce maleate vapors upon processing, which are very lacromatory and therefore unacceptable.
Vinyl halide compositions in dark colors, such as brown, are much more difficult to achieve color, light and heat stability than are light colors, such as white or pastel colors. The dark colors are more prone to be heated by the sun, which in turn has a detrimental effect on stability.
Color change can be measured by exposure in a QUV accelerated weathering tester using a cycle of 20 hours of light at 50.degree. C. and 4 hours darkness with condensation at 40.degree. C. The QUV tester is manufactured by the Q-Panel Company and uses type UV-B lamps with an energy peak at 313 nm. Color change measurements are done in .DELTA.E which is calculated using Friele-MacAdam-Chickering equations as found in the article in Journal of the Optical Society, America, 58, 290 (1968) by G. Wyszecki. The color change for various rigid polyvinyl chloride materials range from about 3.69 to about 51.90 for 350 hours of testing. Color change of lower values of .DELTA.E are desirable to improve the weatherability of polymer materials.
It would be desirable to have a dark color vinyl polymer composition which is stabilized against light, heat and color change, but which does not give off eye irritating vapors during processing.